A linear light-polarizing filter allows light having electric field in a certain plane to transmit at a higher rate than light having an orthogonal electric field. Polarizing filters have been widely used in, for example, ophthalmic products, display devices, imaging devices and optical communication devices. Polarizing ophthalmic lenses are interesting because they have the unique ability to preferentially eliminate glare that is reflected from smooth surfaces such as water and ice.
Dichroic materials have been used for the manufacture of light-polarizing articles. Dichroic materials, when properly oriented, can preferentially transmit light polarized in a particular direction. Dichroic materials can be polarizing over a relatively wide spectrum, such as the visible spectrum, or they may have the polarization property over a narrow range of wavelength.
Light-polarizing articles, such as polarizing ophthalmic lenses, have been manufactured by using various technologies and materials. Most of these products comprise a light-polarizing layer in addition to a non-polarizing substrate. The light-polarizing layer is mostly formed from the dichroic material mentioned above. Current methods for manufacturing polarizing articles have several drawbacks.
One problem is the distortion of the optical surface by the light-polarizing layer in the final product. Some of the light-polarizing articles are currently produced by laminating a pre-formed polarizing layer to a substrate. It is difficult, if not impossible, to allow the pre-formed polarizing layer to curve to a perfect fit with a contoured surface of the substrate, as is in the case of ophthalmic products. The less than perfect fit between the light-polarizing layer and the substrate can lead to undesired cylindrical power in an ophthalmic lens.
Most of the processes used for the manufacturing of polarizing articles are based on gluing or embedding organic polarizing films that must be purchased from polarizing film suppliers. Unfortunately, these processes can only be carried out at the lens-manufacturing site and not in a prescription laboratory. Moreover, films are difficult to be deformed so that they match the curvature radius of high power lenses without optical distortion. Thus, this process is limited to low power lenses
Another technique suitable to prepare polarizing articles is based on the deposition of liquid crystal dyes directly on a substrate. The polarization efficiency being imparted by the alignment of dyes in the parallel grooves made on the substrate is known. The polarizing layer is easily made on the surface of the substrate by means of usual deposition processes regardless, to some extent, of the radius of curvature of the substrate. Among the deposition techniques available, dip coating or spin coating are preferred because they are easily compatible with a lab environment.
Although this method based on the deposition of oriented liquid crystal dyes is particularly convenient, the polarizing layer obtained is very sensitive to scratches due to the poor mechanical properties of organic dye layers. Moreover, the dyes are soluble in water at high temperature even when stabilized by ion exchange using multivalent metal salts such as barium chloride, aluminum chloride, and the like. Therefore such polarizing articles are quickly destroyed by scrapping and do not survive environmental testing such as water test or weathering tests generally required for ophthalmic and sun glass products.
For this reason, there is a need to apply a protective coating on the top of the polarizing layers in order to reinforce their mechanical and moisture resistance and to ensure acceptable durability. However, polarizing articles protected with a layer such as a UV-cured acrylic coating using techniques known in the art are not very durable. There is poor adhesion between the polarizing dye layer and the protective coatings, which may account for the reduced durability. Therefore there is a need for a simple and economically viable process for protecting a polarizing article having a polarizing dye on a support that provides enhanced adhesion between the polarizing dye layer and the protective coating.